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(1) Field of the Invention
The present invention relates to a process for treating moving fibers using high intensity ultraviolet light between 160 and 500 nm without higher wavelengths and ozone. The treatment enhances the strength of the fiber and bondability of the fiber is improved by increasing the oxygen in contact with the surface of the fiber.
(2) Description of the Related Art
In general, manufactured surfaces of substrates, such as the adhereneds in adhesive joints or reinforcing fibers in composites, usually contain undesirable compounds or additives that limit or reduce adhesion to an adhesive or paint film. Hence, surface preparation, which includes cleaning and activation of the surfaces, of polymeric, polymer composite, fibers or metal substrates is carried out prior to applying protective paint films or adhesive bonding. Surface preparation determines the mechanical and durability characteristics of the composite created. Currently, the techniques used for surface preparation are mechanical surface treatments (e.g. abrasion), solvent wash and chemical modification techniques like corona, plasma, flame treatment and acid etching. Each of the existing processes have shortcomings and thus, they are of limited use. Abrasion techniques create dust and abrasive wear waste, are found to be time consuming, labor intensive and have the potential to damage the adherent surface. Use of organic solvents results in volatile organic chemical (VOC) emissions. Wet chemical techniques (such as acid etching) are costly, create waste that must be disposed of or recycled and need tight control. Dry chemical methods (such as plasma etching) are usually batch processes and of limited use with regard to treating three dimensional parts.
The use of lasers for surface treatment is known in the art. The focused beams of the lasers make it difficult to treat a large surface or non-regular surfaces such as fiber tows containing thousands of fibers. U.S. Pat. No. 4,803,021 to Werth et al describes such a method. U.S. Pat. No. 4,756,765 to Woodroffe describes paint removal with surface treatment using a laser.
Plasma treatment of surfaces is known in the art. Relatively expensive equipment is necessary for such treatments and plasmas are difficult to control. The surfaces are treated with vaporized water in the plasma. Illustrative of this art are U.S. Pat. Nos. 4,717,516 to Isaka et al., 5,019,210 to Chou et al., and 5,357,005 to Buchwalter et al.
A light based process which cleans a substrate surface also creates a beneficial chemistry on the surface for adhesive bonding and paintability is described in U.S. Pat. No. 5,512,123 to Cates et al. The process involves exposing the desired substrate surface to be treated to flashlamp radiation having a wavelength of 160 to 5000 nanometers. Ozone is used with the light to increase the wettability of the surface of the substrate being treated. Surfaces of substrates such as metals, polymers, polymer composites are cleaned by exposure to the flashlamp radiation. The problem with the Cates et al process is that the surface of the substrate is heated to a relatively high temperature, particularly by radiation above 500 nanometers and relatively long treatment times. Related patents to Cates et al are U.S. Pat. Nos. 3,890,176 to Bolon, 4,810,434 to Caines; 4,867,796 to Asmus et al; 5,281,798 to Hamm et al and 5,500,459 to Hagemeyer et al and U.K. Patent No. 723,631 to British Cellophane. Non-patent references are: Bolon et al., xe2x80x9cUltraviolet Depolymerization of Photoresist Polymersxe2x80x9d, Polymer Engineering and Science, Vol. 12 pages 109 to 111 (1972). M. J. Walzak et al., xe2x80x9cUV and Ozone Treatment of Polypropylene and poly(ethylene terephthalate)xe2x80x9d, In: Polymer Surface Modification: Relevance to Adhesion, K. L. Mittal (Editor), 253 to 272 (1995); M. Strobel et al., xe2x80x9cA Comparison of gas-phase methods of modifying polymer surfacesxe2x80x9d, Journal of Adhesion Science and Technology, 365 to 383 (1995); N. Dontula et al., xe2x80x9cA study of polymer surface modification using ultraviolet radiationxe2x80x9d, Proceedings of 20th Annual Adhesion Society Meeting, Hilton Head, S.C. (1997); C. L. Weitzsacker et al., xe2x80x9cUtilizing X-ray photoelectron spectroscopy to investigate modified polymer surfacesxe2x80x9d, Proceedings of 20th Annual Adhesion Society Meeting, Hilton Head, S.C. (1997); N. Dontula et al., xe2x80x9cUltraviolet light as an adhesive bonding surface pretreatment for polymers and polymer compositesxe2x80x9d, Proceedings of ACCE""97, Detroit, Mich.; C. L. Weitzsacker et al., xe2x80x9cSurface pretreatment of plastics and polymer composites using ultraviolet lightxe2x80x9d, Proceedings of ACT""97, Detroit, Mich.; N. Dontula et al., xe2x80x9cSurface activation of polymers using ultraviolet activationxe2x80x9d, Proceedings of Society of Plastics Engineers ANTEC""97, Toronto, Canada. Haack, L. P., et al., 22nd Adhesion Soc. Meeting (Feb. 22 to 24, 1999).
Non-pulsed, low intensity UV lamps have been used by the prior art. These are described in: xe2x80x9cExperimental Methods in Photochemistryxe2x80x9d, Chapter 7, pages 686 to 705 (1982). U.S. Pat. No. 5,098,618 to Zelez is illustrative of the use of these types of lamps. The UV light is preferably at 185 and 254 nm wavelengths in the presence of oxygen which generates atomic oxygen and ozone. The result is that the surfaces are more hydrophilic. The treatments are for 5 to 120 minutes which is a relatively long time. The lamps are low pressure and have an intensity of 10 to 15 m-Watts/cm2. The substrate is a distance of no more than 1.25 cm. The lamps are relatively low power and thus require a long treatment time.
There are multiple prior art processes for the surface treatment of carbon fibers. Carbon fibers are routinely used in aerospace, automotive, recreational, and durable goods markets. Three major methods are currently practiced: 1. Anodic oxidation, a wet process where the proper voltage and electrolyte solution concentration must be maintained; 2. Exposure to ozone gas at elevated temperature, where ozone concentration and temperature must be maintained; and 3. Treatment in caustic solutions such as nitric acid, where solution strength and treatment time must be maintained. Organic polymer fiber composites are experiencing 15% annual growth and a new method of fiber surface treatment could find applications in both fiber-thermoset and fiber-thermoplastic matrix composites.
U.S. Pat. No. 3,723,607 to Kalnin describes a process wherein carbon fibers are heated to 900 to 1400xc2x0 C. in an inert atmosphere and then subsequently heated in ozone at 75xc2x0 to 175xc2x0 C. for a period of time of at least 30 seconds. The process is expensive because of the heating steps. The process provides oxygen on the surface of the fibers and improved composite strength.
U.S. Pat. No. 3,754,957 to Druin also shows the need for treating the surface of carbon fibers with oxygen. The fibers are heated at 1000xc2x0 C. to 1800xc2x0 C. in the presence of oxygen. The fibers form better composites.
U.S. Pat. No. 4,832,932 to Tada et al describes treating fibers to increase the oxygen content of the surface. Various methods are described including heating in ozone at 200xc2x0 C. This patent also shows the need for providing oxygen in the surface part of the fibers.
Thus, there is a clear need to provide an oxygenated carbon fiber surface which provides for the preparation of improved composites. The problem is that the prior art processes require considerable amounts of energy, create waste products, require careful control and thus are expensive. There is a need for an improved process.
A disadvantage of the ultraviolet lamp fiber treatments of the prior art is that they are time consuming and sometimes unreliable. To achieve suitable surface chemistries for adhesive bonding and painting purposes, exposure times for certain materials like polypropylene, thermoplastic olefins (TPO""s) tend to be of the order of 5 to 60 minutes. In many cases, there is a limit on the length of time to which one may expose the substrates to UV since there is a fear of degrading the fiber. There is a need for development of an environmentally friendly as well as cost effective and robust surface treatment process which can be used over a range of surfaces.
It is therefore an object of the present invention to provide a process and an apparatus which produces a surface treated fiber with enhanced wettability by matrix or adhesive polymers, and superior bondability to these polymers. In particular, an object of this invention is to modify the surfaces of reinforcing fibers to promote wetting, chemical compatibility, and adhesion between fibers and polymers in composites. The process saves time, conserves energy and is cost effective, reliable and environmentally acceptable compared to current technologies. These and other objects will become increasingly apparent by reference to the following description and the drawings.
The present invention relates to a process for binding oxygen on a surface of a carbon fiber which comprises:
irradiating the surface of the fiber with ultraviolet light at a high intensity between about 1 to 40 watts/cm2 to modify the surface which is moving relative to a source of the ultraviolet light in an ozone atmosphere, wherein the wavelengths are between about 160 nanometers and 500 nanometers without higher wavelengths, wherein the irradiated fiber surface contains the bound oxygen from the ozone.
The present invention also relates to an apparatus for binding a surface of a fiber containing carbon comprising:
(a) an annular light source for irradiating the fiber and, wherein the light source irradiates the fiber at a high intensity between about 1 and 40 watts/cm2 at wavelengths ranging from about 160 to 500 nanometers without higher wavelengths;
(b) a flow directing means for providing flowing ozone on the surface of the fiber during the irradiating by the light source; and
(c) an electrical power supply operably coupled to power the optical energy source wherein the fiber after irradiating contains bound oxygen from the ozone.
The advantages of the present process are: short treatment times which are conducted at ambient temperature and pressure. The process does not involve the use of caustic acids or solutions, is environmentally acceptable, and is amenable to both inorganic and organic fibers such as but not limited to carbon, glass, ceramic, metallic, polymeric, calcium phosphate, aramid, polyolefin, and natural fibers such as jute, henequen, hemp, and others, and nanophase composites. The process is economically competitive or superior to old processes; can be used on continuous or discontinuous fibers.
The basis of operation of the new process is the application of high intensity pulsed, ultraviolet light in the presence of ozone. The fibers to be treated are subjected to short wavelength ultraviolet light, 180 to 500 nm wavelength, in the presence of flowing ozone gas. The ultraviolet light interacts with ozone to create monatomic oxygen, a highly reactive chemical species which is available to react with the fibers. Additionally, the energetic ultraviolet light can interact with the fiber surface to disrupt and change chemical bonds and create favorable conditions for interaction with the ozone and monatomic oxygen. The result of this two-fold process is the rapid oxygenation of the fiber surface that is essential to promote favorable interactions with the matrix in polymer composites. A schematic of one design for the treatment of continuous fibers is provided in FIG. 1.
In the present invention it was discovered that if higher wavelengths of light above about 500 nm are used, the surface of the fiber is heated unnecessarily. A thermoplastic fiber would be heated unnecessarily. This allows the newly formed chemically active groups to rotate into the melted surface thus, decreasing the activity of the surface. On inorganic fiber surfaces, desorption of the functional groups can take place, negating the beneficial effect. By limiting the wavelengths produced by the UV lamp or by the use of filters, the light above 500 nanometers can be largely eliminated.
The process of the present invention is cost effective for pretreatment of surfaces of fibers composed of carbon containing materials such as carbon polymers and polymer composites. The process creates beneficial surface chemistries for adhesive bonding. The advantages of this process over the existing prior art is that the process is cheaper than chemical modification techniques such as plasma and is not a batch process as with plasma and acid etching. The process is environmentally friendly as compared to solvent wash, acid etching and mechanical abrasion techniques. The process is much cheaper than processes using UV exciter lasers which are cost intensive and work on the principle of ablating the surface layers or roughening the surface or amorphizing the top surface layers. In comparison to the existing ultraviolet lamp techniques, the current process reduces the process times for treating fiber surfaces (thus making it less expensive, and avoiding degradation of the substrates) and achieves surface modifications which were not possible. This invention is used to tailor the chemistry of the fiber surface by using ozone between the substrate surface and ultraviolet light. The process can be particularly used for treating carbon fibers prior to their use in composites, in particular. This process is an easier, flexible or alternate way for them to create materials with different surface and adhesion properties to polymer matrices.
The substance and advantages of the present invention will become increasingly apparent by reference to the following drawings and the description.